Manufacturers of internal combustion engines are continually developing new engine control strategies to satisfy customer demands and meet various regulations. One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such engines include both compression-ignition (diesel) and lean-burn spark-ignition engines. When an engine operates in a region lean of air/fuel stoichiometry, a resultant typically includes increased combustion temperatures, leading to increased NOx emissions.
One proposed type exhaust aftertreatment system and control strategy for managing and reducing NOx emissions includes injection of urea into an exhaust gas feedstream upstream of a selective catalytic reduction (‘SCR’) catalytic device to cause a reduction of NOx exhaust gases to nitrogen and oxygen, among other gases.
Effective control of rate of injection of urea is required to reduce engine-out NOx emissions. A urea injection rate preferably matches an engine-out NOx emissions rate for optimum NOx reduction. Typical urea injection control systems currently in use are not capable of controlling urea injection at very low flow rates, which typically occur at engine idle conditions and when a vehicle is decelerating, i.e. conditions when the engine is producing very low engine-out NOx emissions. A typical urea-SCR catalyst has a capacity to store ammonia, which comprises urea which has decomposed on the catalyst surface. The catalyst is able to continue NOx reduction when the urea injection control system is not capable of controlled dosing, i.e., supplying a controlled amount of urea. The stored ammonia reacts with the NOx emissions on the catalyst surface to produce nitrogen, i.e. N2.
There is a need to effectively control urea injection in a powertrain system so equipped.